Models of WO_x films growth during pulsed laser deposition at elevated pressures of reactive gas

The films of tungsten oxides were prepared by pulsed laser ablation of W target in a reactive gas atmosphere (air of laboratory humidity). Optical analysis and ion signal measurements for the laser plume allowed to recognise a threshold gas pressure that suppresses the deposition of non-scattered atomic flux from the plume. When the pressure exceeds about 40 Pa, the films grow due to the deposition of species that could be formed in collisions of W atoms with reactive molecules (e.g., O₂). Kinetic Monte Carlo method was used for modelling film growth. Comparison of the model structures with the experimentally prepared films has shown that the growth mechanism of ballistic deposition at a pressure of 40 Pa could be changed on the diffusion limited aggregation at a pressure of ∼100 Pa. Thus, a cauliflower structure of the film transformed to a web-like structure. For good correlation of experimental and model structures of WO_x, a dimension of structural elements in the model should coincide with W-O cluster size. (paper)